The site of any important dam should be carefully studied by a competent geologist, who should not only be conversant with the geological history of the entire region, but should be afforded time and opportunity to study the immediate vicinity more minutely.
It is well to have in mind that there is usually a geological reason for the location of the stream, i.e., why it is where it is instead of somewhere else. This reason may exist in the rock as a longitudinal fault, or as a stratum of more easily eroded rock, and is much more likely to be uncovered in the bottom of the foundation than it is part way up the side. Stratification far from the horizontal indicates that water must travel by a much longer and more devious course in order to escape under the dam.
It may be taken almost as an axiom that the rock under a river bed is in better condition than the rock exposed above, and will in all probability require less depth of stripping to arrive at a suitable foundation. The rock below water level, besides being more recently exposed by erosion, is protected from the disintegrating influences that are at work above; in fact, under a stream that is actively at work on its rock bed it may be expected that little excavation will be required.
In a large number of cases the rock upon the two sides of the river is different, as when there has been an intrusion of igneous rock through the sedimentary bed rock of the region. The contact of the two rocks is likely to show a zone of weakness, which zone, as it was easily eroded, was searched out and followed by the drainage, thereby resulting in the location of the stream and the formation of the valley or canyon.
A reassuring condition is where the rock upon both sides of the river is the same in kind and position, particularly in the absence of serious distortions or faults. Nevertheless it cannot be too strongly urged that outcroppings afford no safe basis for final conclusions; at best, and with the most intelligent and conservative interpretation, they should be supplemented by core borings and tests of the holes. The geologist should determine the number, location and depth of the borings; and, to facilitate his analysis of results, he should have adequate topographical plans and sections of the site.
The core boring is so called because the cutting is done at the edge of a revolving cylinder, such that a rod or core of rock is preserved inside of the cylinder and can be drawn out with it.